This invention relates generally to radially expandable endoprostheses which are adapted to be implanted in a physiological lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside of a physiological lumen. A “lumen” refers to a cavity of a tubular organ such as a blood vessel or other physiological passageway. A stent, or implantable medical device, is an example of an endoprosthesis. Stents are generally cylindrically shaped devices which function to hold open or expand a physiological lumen, or to compress a lesion. A stent must be able to satisfy a number of mechanical requirements. For example, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of the tubular organ. Accordingly, a stent must possess adequate radial strength.
In adults, primary cancer of the tracheobronchial tree or cancer of the head, neck or chest that extends into the tracheobronchial tree frequently causes lumen compromise and airway obstruction. “Tracheobronchial” refers to the physiological passageway from the throat to the lungs. In some methods of treatment, a compromised component of the tracheobronchial tree can be removed by laser treatment, mechanical debulking, electrocautery, brachytherapy, photodynamic therapy or cryotherapy. A stent can then be placed at the treatment site following removal of a comprised component to maintain the airway lumen to counteract collapse or edema.
Alternatively, a stent can be placed to help compress any lesion extending into the tracheo or bronchi without the need for removal of the compromised component. In some methods of treatment, a stent has been used to palliate patients with inoperable bronchogenic cancer, primary tracheal tumors and metastatic malignancies.
Stents which have been used in the tracheobronchial tree include metal, silicone and bioabsorbable stents. Metallic stents are generally made from an inert metal such as stainless steel, cobalt chromium and Nitinol. Some problems associated with known stent types delivered to the tracheobronchial region include inflammation, stent migration, epithelial damage, granulation tissue formation and mucous plugging. In addition, it is believed that known bioabsorbable stents designed for placement in the tracheobronchial region are not able to adequately combat inflammation caused by stent placement.
“Stent migration” refers to the gradual movement of the stent down the tracheobronchial tree after placement thereof. Stent migration of silicone stents in the tracheobronchial tree is common. “Mucous plugging” is an excessive production of mucous produced in response to the stent. Mucous plugging can cause interference with breathing. “Granulation tissue formation” is the formation of new tissue in response to a wound or other disruption of tissue. Excessive granulation tissue formation can cause a stent to be permanently lodged within a passageway complicating removal if required. Metal stents are especially susceptible to granulation tissue formation. Accordingly, a tracheobronchial stent which addresses these problems is desirable.